The molding cycle performed by an injection molding machine typically comprises the phases of clamp, inject, pack, recover and eject. This invention is principally concerned with the inject phase or injection stroke of the molding cycle.
The study of how plastic flows through the mold is called mold flow analysis. The flow of plastic is critical to a number of factors in the final part including surface flaws and structural integrity. Mold flow analysis predicts the shape of the part at all times as its is being formed and can predict areas of the part where a minimum injection speed is required to fill the part before the gates freeze or perhaps a maximum injection speed above which splaying may cause surface flaws. Experience gained from the use of any particular mold will show where certain speeds are required in mold filling to produce acceptable molded parts. On a production basis, typical practice is to minimize the injection time to achieve maximum machine throughput. All of this is achieved by controlling the injection speed of the ram during mold filling and to, a lesser extent, by the packing phase of the cycle which assures that the mold remains filled with molding material under a desired pressure until the molding material solidifies.
Very early molding machines were operated manually. To inject plastic into the mold, a crank wheel was turned. The faster the wheel was turned, the faster the injection speed. To increase pressure, the crank was turned harder. When crank wheels were replaced with hydraulic systems, the injection was controlled by the flow of oil into the injection cylinder. Various techniques have evolved over the years to control the flow and pressure of the oil. As noted above, controlling the oil flow controls the ram speed and controlling the ram speed controls how the molding material fills the mold. As mold flow analysis matured into a science, the art of injection molding became replaced with decisions dictated by mold flow analysis which required that controls be added to the hydraulic system to control the flow and pressure of the oil. This invention is directed to such a system (although the invention is also applicable to controlling the speed and torque of motors used in "all electric" molding machines).
Initially control systems were simply timers which caused various valve openings at set times during the injection stroke. The timers gave way to micro switches which were tripped by ram movement to cause various valve settings. The micro switches were replaced by feedback position sensors in use today developing signals used by programmable controllers to control ram speed.
The typical injection molding machine most likely observed today has an operator station with a screen display and keyboard which sends signals to a programmable logic controller (PLC). The operator station typically includes a screen whereat the operator can set desired ram velocities at fixed ram travel increments. Typically the ram travel of the machine is divided into ten equal lengths or zones. The operator sets the speed at each zone so that a series of bar graphs are assembled. The ram follows the bar graphs. Recent control improvements have replaced the bar graphs with points at each zone boundary so that the ram is not programmed to travel at constant speed within each zone but at a speed which constantly varies from one set point at one zone boundary to another set point at an adjacent zone boundary. When the operator makes a number of desired velocity settings at set ram travel positions which the ram is to follow, he establishes a "velocity profile". The object of the control system is to actually cause the ram to travel at the user set speeds at the user set positions i.e., to emulate the velocity profile. As will be explained in the Detailed Description of the Invention below, this invention causes better control of the ram speed than what heretofore has been possible.
The control within the PLC which causes the ram to follow the velocity profile is typically a PID controller (proportional/integration/derivative controller). A PID controller receives a velocity feedback signal from a sensor on the machine and compares it with the user set velocity control signal to generate an error compensated control signal by which the machine's speed is controlled. The control signal is then converted to an analog driving signal controlling a solenoid valve regulating a hydraulic proportioning valve in turn controlling flow from a pump to a prime mover causing ram movement. The PID controller is the typical mechanism for achieving closed loop control. It is generally used because the machine's control modules are typically purchased by injection molding machine manufacturers from control suppliers who assemble control systems for special applications, such as injection molding machines, from any number of common control modules having desired response times, sensitivities, robustness, etc. The systems use a common control, such as the PID controller.
The set points entered by the operator are turned into velocity control signals after the machine has been set up and calibrated. Functionally, the user entered set points at the machine console are converted into set point velocity signals by the PLC and outputted at ram travel positions as analog drive signals. The PLC uses the PID controller to provide closed loop control (vis-a-vis ram position sensor signals) to assure that the proper set point velocity signal, corrected for error, is outputted as the drive signal. To prevent the PID controller from having to generate large error corrections, normal practice is to manually calibrate the proportioning valve for set point velocity signals. The solenoid voltage is manually adjusted until movement of the ram is visually detected and the analog signal causing this initial movement is saved in digital form as the valve offset. The solenoid voltage is then manually adjusted to a value whereat maximum rated speed of the ram is observed to occur. For example, if maximum ram speed for the machine is one inch per second and rise travel was five inches, a technician, using his watch, would manually adjust solenoid voltage until he was able to cause the ram to travel five inches in about five seconds. At that point, the span voltage in digitized form would be entered. By assuming a straight line relationship between offset and span a digital signal corresponding to a user set velocity anywhere between zero and one inch per second is calculated by the machine's control. If the machine has an energy savings mode, a second calibration must be done. This technique is not precise. However, conventional thinking is that whatever errors are produced, they can be addressed in the PID control and calibration can only be done without load. Thus, the settings are made to simply assure capacity of the machine.
The PID control, however, has to be tuned. While factory default settings are, of course, provided, the tuning is done through a trial and error procedure by the molding machine manufacturer's technician during machine set up. Basically, the procedure followed after setting span and effect is to boost the ram during the injection stroke and observe the speed response. Various "art form=" techniques are used to adjust the factory settings if the response is deemed sluggish.
Many current machines allow the operator to select open loop or closed loop control to achieve velocity profiling. The open loop mode is achieved by simply using the manually calibrated valve settings set during machine set-up as described. Closed loop is achieved through the tuned PID controller as described or by using the default factory settings for the PID controller. In practice, it often occurs that the PID loop is out of tune with wear and age or if the molder simply changes the molded part. The machine user does not have the sophistication to re-tune the PID loops and closed loop control does not follow the velocity profile. In fact, in many cases the machine with the control in open loop will more closely follow the velocity profile even though, assuming perfect calibration of the proportioning valve, the open loop control cannot account for the load and specifically the disturbances or resistances imposed by the melt on the ram during the injection stroke.
The above summarizes, to a good part, what the assignee of this invention has observed in the marketplace with respect to current control systems (apart from assignee's control system which is the subject of this invention). The literature has disclosed, however, a number of control techniques applied to injection molding control systems.
U.S. Pat. No. 5,645,775 to Spahr et al.; U.S. Pat. No. 5,258,918 to Giancola; U.S. Pat. No. 5,182,716 to Stroud, III et al. and U.S. Pat. No. 5,062,785 to Stroud, III et al. disclose control systems which have specific features for controlling the ram velocity. In these systems, the velocity profile is broken down into zones as discussed above. In order to transition smoothly from one zone to the next zone, the controls transition from open loop to closed loop within a zone. Additionally, there is discussed feed forward but in the sense of feeding a present signal ahead in time and not a predictive signal. Additionally there is disclosed adaptive learning concepts. The latter techniques are well known in control theory and the references simply show that they have been applied to control systems for injection molding machines.
U.S. Pat. No. 5,482,662 to Nakamura et al. discloses a somewhat more sophisticated approach to feed forward control for ram velocity in that pressure sensed by the valve is used to develop a feed forward signal to account for latency response of the valve and another control term is added in when the ram position feedback signal reaches a set differential ratio to the set velocity term. The valve control is said to eliminate overshoot tendencies of the valve resulting from changing signals and the velocity feedback contributes to accuracy during steady state. The Nakamura system appears to be more advanced than the concepts earlier discussed but still uses an additional term switched in or out of the control during injection depending on feedback of a current event.
U.S. Pat. No. 5,578,256 to Austin utilizes the relationship between ram velocity and mold flow to develop a plastic flow characteristic, i.e., pressure, sensed in the mold during a run which is then inputted into the control as an adaptive error term in the next succeeding cycle.
U.S. Pat. No. 4,753,588 to Kiya and U.S. Pat. No. 5,552,690 to Hiraoka relate to electric drive injection molding machines controlling ram speed. Kiya discloses using a feed override look up table to modify the set speeds. Hiraoka uses separate control terms to control motor torque and motor speed. Feedback control at the zone boundaries is used to adjust the torque and velocity settings.
None of the systems discussed appear to utilize feed forward concepts in the predictive sense as disclosed in my U.S. Pat. No. 5,493,503 and this invention may be viewed as an extension of and an improvement to that patent. Further the systems are generally geared to sensing events at zones and making changes during zone progressions by switching control modes or simply by adding adaptive, learned error signals. None of the systems cited discuss the set point signals. They simply generate the set point signals corresponding to the user set points and then utilize feedback techniques to produce the desired ram control.